The present invention relates to an image display method and an image display apparatus and more particularly to an image display method and an image display apparatus for displaying on a color display a monochromatic image having grayscales larger in number than the drive grayscales of the image display section.
A diagnostic image picked up by a medical diagnostic apparatus such as an X-ray diagnostic apparatus, an MRI (magnetic resonance imaging) diagnostic apparatus, or various CT (computed tomography) apparatuses is generally recorded on a light transmissible image recording film such as an X-ray film or other film photosensitive materials and is reproduced as a light transmissible image. The film on which the diagnostic image is reproduced is set in an observation apparatus called a viewing box, is observed in a state that light is irradiated from the rear thereof, and is diagnosed of existence of a lesion.
Further, to various medical diagnostic and measuring apparatuses, as a monitor for observing a picked-up and measured image, a color display such as a CRT (cathode ray tube) display or an LCD (liquid crystal display) is connected and by images outputted to these display screens, a diagnosis, confirmation and adjustment of a diagnostic image before outputted to the film, and an image process are performed.
Meanwhile, when reproducing the image picked up by the aforementioned X-ray diagnostic apparatus on a film, generally, a blue-based monochromatic film is used often. Further, generally, an image is reproduced often by a grayscale resolution 10 to 12 bits long (1024 to 4096 grayscales).
Therefore, also when diagnosing an image on a display such as a CRT or an LCD, an exclusive monochromatic display having a grayscale resolution more than 10 bits long is used often.
On the other hands, to display a color image by an endoscope or an eyeground camera, a color display is used. Further, in recent years, also when displaying a three-dimensional image by an ultrasound diagnostic apparatus, a CT apparatus, or an MRI apparatus, a color display has been used.
To make a synthetic diagnosis, it is necessary to observe images of a plurality of kinds of diagnostic apparatuses and for that purpose, both exclusive high-grayscale monochromatic and color displays must be installed and a problem arises that it is expensive and a wide installation space is required.
A color display can display a monochromatic (black and white) image, though on the color display, an image is displayed generally at an 8-bit grayscale resolution, so that when reproducing an image on an ordinary display screen, an image is displayed by the so-called bit-down image data having a lower grayscale resolution than that of an image picked-up and outputted by the aforementioned X-ray diagnostic apparatus.
Concretely, for example, to convert 10-bit monochromatic image data to 8-bit R, G, and B image data, a monochromatic image signal value of 1024 grayscales, on the basis of an LUT (look up table) as shown in FIG. 19, is converted to R, G, and B values of 256 grayscales. Here, in the conventional LUT, the R, G, and B values are all equivalent and a problem arises that the R, G, and B image data cannot display an image of grayscales more than 256 grayscales.
Furthermore, as in the invention described in Patent Document 1 (Tokkai 2000-330530, Japanese Non-Examined Patent Publication), an image display apparatus for making the B value in the LUT larger than the R and G values, thereby reproducing a blue-based monochromatic film is known. According to such an image display apparatus, an LUT is prepared under the condition of R value=G value=K×B value (0<K<1), thus the color tone of the blue-based monochromatic film can be reproduced almost, though the maximum values of the R and G values become smaller than 256, so that the number of displayable grayscales becomes smaller than 256, thus when reproducing the color tone of the blue-based monochromatic film, the reduction in the number of grayscales is a bigger problem.
As a method for displaying grayscales more than the number of drive grayscales of the display, a method of FRC (frame rate control) display is devised.
Here, the FRC display device that when displaying image data having a high grayscale resolution (the number of bits) as image data having a low grayscale resolution (the number of bits), from the image data having a large number of bits, image data having a small number of bits which is the number of bits in accordance with the difference in the number of bits between both image data is prepared, and the image data is displayed sequentially, thus a grayscale equivalent to the large number of bits is represented by image display of the small number of bits.
Concretely, assuming the difference in the number of bits as n, image data of a small number of bits of the number of frames 2n is prepared, and the image data of the small number of bits is displayed sequentially, thus for example, using 4 frames of images of an 8-bit grayscale resolution, a grayscale equivalent to a 10-bit grayscale resolution is represented.
Further, as in the invention described in Patent Document 2 (Tokkai 2001-034232, Japanese Non-Examined Patent Publication), an LUT is prepared so that not only the R, G, and B values are equivalent but also as shown in Table 1 of Patent Document 2, the R, G, and B values are not reduced monotonously and the total of the R, G, and B values is changed one by one, thus an image of 765 grayscales can be displayed.
Furthermore, as in the invention described in Patent Document 3 (Tokkai 2003-050566, Japanese Non-Examined Patent Publication), an image display apparatus for representing a multi-grayscale using an LUT for changing a sub-pixel signal value within an optional range is known. According to such an image display, theoretically, an image of 4096 or more grayscales can be displayed.
However, in the FRC display, a problem arises that image flickering which is called the so-called flicker is conspicuous and the eyes are made tired and another problem arises that the burden imposed on the process required to change the divided display data in the FRC representation is great.
Further, in the image display apparatus described in Patent Document 2, the maximum value of an input monochromatic image signal value is converted to the “maximum value of R+maximum value of G+maximum value of B”, and the R, G, and B signal values are divided and distributed almost evenly, and the restriction to the combination of the R, G, and B values is strict, so that unless the FRC display is performed, only an image of 766 grayscales is represented, thus the image display apparatus is insufficient to a simple pick-up image diagnosis.
Furthermore, in the image display apparatus described in Patent Document 3, a multi-grayscale display on a monochromatic monitor is supposed, so that the selectable range of a sub-pixel signal value is excessively large and the sub-pixel signal value is selected only under the condition concerning luminance, thus even if a monochromatic image is displayed on a color monitor using a prepared LUT, a problem arises that it cannot be displayed with a color tone suited to diagnosis. Further, it is described that the image display apparatus described in Patent Document 3 can be applied also to the color monitor, though in that case, the pixels of R, G, and B must be divided furthermore into sub-pixels and a problem arises that the constitution is complicated.